(1) Field of the Invention
The present invention relates to an image forming apparatus that forms an image on a rotative image carrier and transfers the image formed on the image carrier to an object, and to a lubricant application method for applying a lubricant to the circumferential surface of the image carrier.
(2) Related Art
Electrophotographic image forming apparatuses such as printers are configured to electrically charge a photosensitive drum as an example of a rotative image carrier, form an electrostatic latent image by exposing the charged photosensitive drum to light, form a toner image by developing the electrostatic latent image formed on the photosensitive drum, transfer the toner image thus formed onto a recording sheet, and remove a residual material such as toner on the photosensitive drum not transferred to the recording sheet, by using a cleaner.
In such an image forming apparatus, it is common that a lubricant made of zinc stearate for example is applied to the photosensitive drum in order to improve the performance in transfer, cleaning, and so on.
Japanese Patent Application Publication No. 2008-89771 discloses a structure in which: a cleaning device and a lubricant applying device are located in this order around the rotative photoreceptor along the rotational direction thereof; and in the lubricant applying device, a rotative transport brush is brought into contact with a powder lubricant contained in a casing disposed below the transport brush, thereby picking up the lubricant, and the transport brush transports the lubricant to the location facing the photoreceptor, and thus applying the lubricant to the circumferential surface of the photoreceptor.
When the transport brush as described in the publication above is used, a large portion of the powder lubricant picked up from the casing by the transport brush is held by the top of the bristles of the brush, and is brought into contact with the circumferential surface of the photoreceptor, and is thus transported to the circumferential surface of the photoreceptor. However, there also is a portion of the powder lubricant that remains on the bristles of the transport brush, entering deep into the bristles and staying at the bottom of the bristles.
Considering this, the amount of lubricant to be applied to the circumferential surface of the photoreceptor (i.e. supply amount) per rotation of the transport brush can be obtained by subtracting the amount of lubricant staying in the brush (i.e. accumulation amount) from the amount of lubricant picked up by the transport brush (i.e. consumption amount). When the relationship between the supply amount, the accumulation amount, and the consumption amount is constant, a certain amount of lubricant would be allowed to be stably supplied to the circumferential surface of the photoreceptor over a long period.
In reality, however, the amount of lubricant staying deep in the transport brush increases as the cumulative count of rotations of the brush increases. When the bristles are clogged up with lubricant, it is unlikely that a further portion of lubricant enters deep into the brush. The lubricant that cannot enter the brush is only allowed to temporarily stay at the top of the bristles, and is then supplied to the circumferential surface of the photoreceptor.
Therefore, out of the picked-up lubricant (i.e. consumption amount), the amount of lubricant staying in the brush (i.e. the accumulation amount) decreases as the number of the image formation operations that have been executed increases, and accordingly, the amount of application of the lubricant to the circumferential surface of the photoreceptor (i.e. the supply amount) increases. This increase in the amount of supply to the circumferential surface of the photoreceptor progresses gradually over a long period such as several months, and leads to an excess supply of the lubricant.
The excessive lubricant supplied onto the circumferential surface of the photoreceptor is transported to the cleaning device according to the rotation of the photoreceptor. When, for example, the lubricant passes through the point that faces the developing device located around the photoreceptor, there is a high possibility that a portion of the lubricant is mixed into the developer within the developing device.
If a large amount of lubricant is mixed into the developer, it becomes more likely that a developing failure occurs.